Issue description

Consider the following tmux.conf "base" configuration file:

var1=foo
source -qF '#{d:current_file}/optional.conf'
display "var1: $var1"
display "var2: $var2"

Then consider the optional optional.conf configuration file:

var1=bar
var2=fizz

When running tmux -vv -L test /tmp/tmux.conf, I get

tmp/tmux.conf:3: var1: foo
/tmp/tmux.conf:4: var2:

The manual says

Outside of quotes and inside double quotes, these replacements are performed:

• Environment variables preceded by $ are replaced with their value from the global
  environment (see the GLOBAL AND SESSION ENVIRONMENT section).

Environment variables may be set by using the syntax ‘name=value’, for example ‘HOME=/home/user’.
Variables set during parsing are added to the global environment.

After tmux has launched, the global environment indeed contains the right values:

$ tmux showenv -g var1
var1=bar

$ tmux showenv -g var2
var2=fizz

Interestingly, when using the #{name} syntax, I get the desired behavior.

var1=foo
source -qF '#{d:current_file}/optional.conf'
display "var1: #{var1}"
display "var2: #{var2}"

$ tmux -L test /tmp/tmux.conf
/tmp/tmux.conf:3: var1: bar
/tmp/tmux.conf:4: var2: fizz

→ Is this something that should be fixed or documented?

Required information

• tmux version (tmux -V): tmux 3.5a.
• Platform (uname -sp): Darwin arm.
• Terminal in use (xterm, rxvt, etc): Ghostty.
• $TERM inside tmux (echo $TERM): tmux-256color.
• $TERM outside tmux (echo $TERM): xterm-256color.
• Logs from tmux (tmux kill-server; tmux -vv new).
  tmux-client-57016.log
  tmux-out-57018.log
  tmux-server-57018.log